Trends in Food Science & Technology 14 (2003) 478–481
EHEDG Update
Design of
mechanical seals
for hygienic
and aseptic
applications
This article is an extended summary of the report prepared
by the Mechanical Seals Subgroup of the European Hygienic Equipment Design Group (EHEDG). Originally published
in August 2002, it is the 25th in the series of EHEDG summaries published in TIFS. The full report was prepared by G.
Anderberg (Chairman), R. Cocker, R. Curiel, R. Feenstra, W.
Mathes, H van Eijk and R. Wallis and is available from
CCFRA at pubs@campden.co.uk. Information about EHEDG
can be found on the website at www.ehedg.org. The production of EHEDG guidelines is supported by the European
Commission under the Quality of Life Programme, project
HYFOMA (QLK1-CT-2000-01359). For application in the
field, it is strongly recommended that the guideline be read
in its entirety.
# 2003 Published by Elsevier Ltd.
Introduction
Inappropriate design of mechanical seals can cause
contamination of a food product by micro-organisms,
and/or residues, which may be toxic or allergenic. The
main contamination risks are:
Micro-organisms enter the product from the
atmospheric side through the seal gap.
Product passes the seal gap to the non-product
side and micro-organisms start growing in the
residues. These micro-organisms then re-enter
the product.
Product sticks to the seal or the seal chamber and
micro-organisms start growing.
In addition, cleaning of equipment should not cause
environmental problems and stagnant zones or small
0924-2244/$ - see front matter # 2003 Published by Elsevier Ltd.
doi:10.1016/S0924-2244(03)00159-6
gaps between equipment parts must be avoided. These
risks warrant the setting of more stringent requirements.
This guideline compares the design aspects of different
mechanical seals with respect to ease of cleaning,
microbial impermeability, sterilizability or pasteurizability. This document is intended to serve as a guide for
suppliers and users of mechanical seals in the food
industry.
Classification
Using EHEDG definitions, mechanical seals are classified into three categories, according to their use in the
food industry: Aseptic, Hygienic Equipment Class I,
and Hygienic Equipment Class II (see Table 1). This
guideline covers both single and dual mechanical seals
under the first two categories, which by definition, are
subject to more stringent hygienic demands. For Class
II seals less stringent hygienic demands are needed and
are therefore not included in this guideline.
Basic principles
In closed food processing equipment, the mechanical
seal is capable of reducing the risk of microbial contamination in a wide variety of products under very
demanding operating conditions. For this reason,
mechanical seals are commonly in use for pumps, agitators, mixers, and other types of rotary equipment.
Dynamic sealing takes place at the interface between a
stationary seal ring and a rotating ring. It is accomplished by using perfectly flat face surfaces, and using
the product pressure along with spring force to press the
components together. Typically, a microscopic film of
product fluid between the faces of the rings acts as the
sealant. This fine liquid film separates the two faces, and
conflicts with the general requirement of a crevice-free
design for the food industry. In principle, it cannot be
cleaned in place (CIP) properly when the shaft stands
still. Under normal operating conditions, the seal has a
certain transfer of liquid from the high-pressure side to
the low pressure side of the seal gap.
In a single seal this will stay as a residue on the
non-product side of the seal face.
In a flushed single seal, this residue will be
flushed away with the flushing fluid.
In a dual-seal with higher pressure in the barrier
fluid it will be this fluid transferred over the seal
EHEDG Update / Trends in Food Science & Technology 14 (2003) 478–481
gap into the product. Also, the temperature of
the flushing liquid will secure a safe barrier
between product and atmosphere.
General design criteria (Fig. 1)
The parts of the seal in contact with product
must be suitable for CIP and SIP according to
EHEDG guidelines.
The non-product side of the seal shall be cleanable
and capable of being disinfected and shall not contaminate or have any adverse influence on the food.
Product-contact surfaces have to be smooth and
crevice free.
The non-product side must be as smooth and as
crevice free as possible.
Dead ends are not permitted on the product side.
Springs are not permitted on the product side.
Designed corners must have a minimum radius of
3 mm.
Single mechanical seals
The single seal is most common. It provides some
risks of contamination from the atmosphere. This can
be handled by regularly inspecting the non-product side
of the seal and cleaning manually according to the
HACCP programme. Alternatively the non-product
side of the seal can be provided with a flushing device to
minimize the risk for microbial growth in a humid
environment.
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Dual mechanical seals
When processing sterile products, no ingress of microorganisms is allowed and the equipment has to be in
accordance with the EHEDG class: ‘aseptic’. The single
seal does not meet the requirements of aseptic processing.
A solution to this problem is to provide a flush at
the non-product side of the seal, using a fluid with
bactericidal properties, or another sterile barrier system.
For these applications, dual mechanical seals are used.
Such seals prevent product contamination and avoid
product leakage to the atmospheric side. In some
applications (e.g. top-driven agitators or hazardous
product) the liquid between the seals is applied at a
higher pressure than the product.
Component materials
Materials used for components in mechanical seals
must:
Be non toxic
Be anti-corrosive
Be crevice free
Be non-absorbent
Not transfer undesirable odours, colours or
taints to the product
Not contribute to the contamination of food nor
have any adverse effects on the food.
Have surfaces and coatings which are durable,
cleanable and capable of being disinfected,
without breaks, resistant to cracking, chipping,
flaking and abrasion and prevent penetration of
unwanted matter.
Table 1. Classification criteria for mechanical seals
Class/demand
Description
Aseptic equipment
High demand
Equipment can be cleaned in-place (CIP) and freed from micro-organisms and spores (sterilisation in-place, (SIP))
without any dismantling. A sterile barrier is needed between the product and the atmospheric side. Normally this
is achieved by using two seals between which a sterile medium is present. This medium can be a liquid e.g. steam
condense, or a gas. The barrier chamber has to be clean during production and cleaned during CIP by a flow.
The barrier can have a lower or higher pressure than the product. The sterile barrier medium must be
food-compatible.
Hygienic equipment Equipment that can be cleaned in-place (CIP) and freedfrom micro-organisms and spores without any dismantling
(SIP). Rotating and stationary mechanical seal parts with smooth and crevice-free product contact surfaces. All
Class 1
components with, or forming unavoidable crevices or gaps, e.g. springs, torque transmitters, and shall be located
Average demand
at the non-product side. In addition to the main seal on the product side a secondary sealing device is needed on
the atmospheric side, e.g. a lip seal or another mechanical seal. To keep the non-product side of the seal clean,
this sealing chamber can be flushed with a fluid acceptable for the application.
Hygienic equipment Equipment that is cleanable after dismantling and can be freed from relevant micro-organisms (SIP) after
re-assembly. The seal has to be dismantled for cleaning regularly. The need for hygienic design is not as stringent
Class II
as for Class I equipment. The materials used in the seal must be acceptable for food contact application, as for
Low demand
Class I equipment.
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EHEDG Update / Trends in Food Science & Technology 14 (2003) 478–481
All materials for food application must be compliant
with FDA CFR 21 or equivalent European legislation.
This can take the form of an affirmation statement from
the FDA, that the material is generally recognized as safe
(GRAS statement). It is the supplier’s responsibility to
hand over such a statement for food production equipment on request. Stainless steel, aluminium-oxide and
silicon carbide have such a general GRAS statement.
als shall be smooth and free of flash lines, of low porosity, without blisters and tears at any surface in contact
with the product-wetted area. The material should be
selected according to the requirements of the responsible certified bodies such as FDA and BGVV or evolving European Legislation. Only plastic materials
produced from monomers according to EU Directive
(The Plastic Directive) and subsequent amendments
shall be used.
Seal face materials
The current regulations for materials for use in food
applications tend to use the broad statement that they
should be ‘pore free’. In the materials used for mechanical
seal faces (e.g., carbon, ceramics) this is not strictly true.
These materials are inherently porous as a result of the
material manufacturing process. This porosity is, to a certain extent, positive for seal face lubrication. A method for
the determination of surface porosity is standardized in EN
993-1. The surface finish of the component will affect this.
Current machining procedures ensure good surface
properties (e.g., turning/finishing, grinding, lapping and
polishing). It is important that the materials are compatible with the product and cleaning liquids in use.
Otherwise the surface properties are affected negatively.
Elastomer materials
The secondary sealing elements consist of elastomeric
or plastic materials (e.g., O-rings, gaskets). The materi-
Metal materials
The metal components shall be made of austenitic
stainless steel to a specification corresponding to AISI
type 316 or EN 10088 Part 1. The cast equivalents of
this grade will be permitted provided that the composition is such as to ensure that the structure is fully
austenitic. Sintered metal components, because of
their inherent high levels of porosity, will not be
permitted.
Other metals generally accepted as showing superior
chemical resistance to type 316 stainless steel, for
example, Duplex stainless steels, super austenitic stainless steels, corrosion-resistant nickel alloys and titanium, will also be acceptable. As the springs may be
exposed to aggressive fluids, e.g., cleaning agents, they
must be made of corrosion resistant materials. In this
case stainless steel or even better Hastalloy springs are
to be preferred.
Fig. 1. Design principle for mechanical seals.
EHEDG Update / Trends in Food Science & Technology 14 (2003) 478–481
Parts/components and their hygiene implications
Elastomeric seal elements
Both the dynamic seal ring (taking up some axial
movements) and the static seal ring need to be adequately sealed to either the rotor shaft or the housing.
With specially shaped elastomeric sealing elements the
demand of a gapless design can be realized, although
the above remains valid and must be considered to
ensure proper functioning.
Metal and elastomeric bellows seals
Bellows in general have the advantage that they need
not overcome any friction forces and that they are less
sensitive to leakage deposits on the atmospheric side of
the seal; this means that their axial motion is better than
that of O-rings. The bellows are usually in direct contact
with the product.
Because of the large surface area in contact with the
product when using elastomeric bellows, a smooth and
pore-free surface needs to be assured. Additionally,
deformations under influence of temperature and pressure may be a problem. An elastomeric bellows seal is
normally energized by a spring which fits between the
shaft and elastomer bellows.
Welded metal bellows should not be used due to their
narrow and deep gaps between the metal disks. Metal
rolled bellows can be used with low viscosity products
to overcome stagnant product areas.
Location and housing of springs
For adequate cleanability, springs should not be
exposed to product and must therefore be at the nonproduct side.
Installation requirements
Mechanical seal design
The seal should be positioned in the product flow to
prevent adhesion of product particles and avoid dead or
stagnant zones. This will improve the cleanability by
CIP and help to take frictional heat away from the seal,
thus minimizing coagulation of proteins. Outboard seal
designs are not acceptable if they have an annular dead
end on the product side of the seal. The springs have to
be on the non-product side to prevent product from
sticking and to improve CIP.
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Continuous flushing
A flushed seal is of benefit in keeping the seal clean
(instant removal of product), and avoiding dry startups. This will prolong the life of the seal. Inadequate
control of the seal flush system could therefore cause
seal failures. The flush provides a means to dilute the
leaking product into the flush liquid (sometimes called
barrier fluid, or quench) instead of into the atmosphere.
It requires uninterrupted replenishment of the sterile
flush liquid together with monitoring and maintenance
to ensure faultless operation of the flushing system. This
flow has to be good enough to remove particles from the
flushing chamber.
Aseptic operation
Aseptic operation requires a sterile barrier between
product and atmospheric side of the equipment. In aseptic applications, the flushing medium itself forms the
sterile barrier. Often condensate made from food-grade
steam is passed between the seals for this purpose. The
temperature of the steam condensate during production
is typically 70–135 C and during sterilization, 125–
145 C. Flow rates of condensate are typically 0.1 l/min.
Cartridge design
In general all mechanical seals can be integrated in
pre-mounted units, ready to be installed, including
sleeves, sealing housings, glands, etc., known as cartridges. The cartridge seal prevents damage to the seal
faces during the assembly/disassembly operation of the
food equipment. It has a built-in sleeve to fit an existing
shaft. It allows for easy seal installation because the seal
can be assembled and pre-tested prior to installation.
The ways of mounting the seal requires attention to
assure hygienic installation. It is important to ensure
correct fitting of the seal and its cleanability. Edgemoulded rubber gaskets (or metal–rubber bonded seals)
maintain a sufficiently tight seal, because the rubber will
not compress past the metal carrier thickness. Cartridge
designs are to be preferred whenever possible.
The mounting and static sealing of the cartridge in its
adjacent machinery environment should be designed
considering hygienic/aseptic aspects. This means for
example no screws or threads in contact with the product and the static sealing with a special encapsulated
O-ring or elastomeric edge-moulded gasket.
Flushing system on non-product side
It is important that there is a controlled flow around
all surfaces in the flushed chamber, particularly at the
non-product side of the seal faces. The flushed area shall
be as smooth and free of crevices as possible to prevent
adhesion of residues.
The flushed area must be fully drained when the
flushing is off (a small amount of water will vaporize
during SIP).
Definitions
For the purposes of this guideline, the definitions
given in ENV 1070 apply. A full list of definitions is
given in the full document.
References
Please refer to the original document for references on
legislation/guidelines mentioned in this summary.